This invention is a process and device to compensate a voltage variation between the electrodes of the tube of a microwave amplifier so as to diminish the phase variation of the amplified wave and thereby ensure its stability.
It applies notably to improving the phase stability of microwave tube radar transmitters when they are subjected for example to large variations of the duty cycle of the transmitted waveform. More generally, it applies to all types of microwave tube amplifier subjected to large load variations.
Phase instability is preponderant among the various types of instability affecting the operation of microwave tube amplifiers. It alters the quality of the transmitted wave and in particular prevents Doppler processing, known to the expert, using measurements of phase differences between the wave transmitted and the wave received by a radar. These phase instabilities are caused by variations of the voltage between the cathode and the body of the tube. In the case of a klystron or of a travelling wave tube, for example, a 1% variation of this voltage causes a phase rotation of approximately 20.degree. and 40.degree., respectively. The voltage variations between the cathode and body of the tube of the transmitter are caused by more or less sudden variations of the load on the terminals of the power supply unit delivering the voltage. Such load variations occur, for example, during a transition from the silence condition to full-power transmission, or more generally when there are large variations of the duty cycle of the transmitted waveform, representing strings of patterns or bursts at very different mean values.
A first known solution that decreases these phase rotations consists of very precisely regulating the voltage between the cathode and body of the tube to limit these variations to approximately 1% of the nominal voltage, for example. To accomplish this, this solution makes it necessary in particular to have recourse to power supply units employing a high switching frequency with very advanced regulation systems having a wide pass band and a high gain. But the nominal voltage is often fixed in a range extending from a few kilovolts to many tens of kilovolts, further complicating the building of these power supply units and, more importantly, making them very expensive, in particular because they require circuits and components that are very precise and can withstand very high voltages, and great precautions in adjustment.
This first solution being set aside for reasons of cost, another known solution phase-locks the signal amplified by the transmitter to the signal the transmitter receives, making it possible to use less sophisticated and therefore less expensive power supply units. However, in this case it is impossible to obtain stable operation of the phase lock loop with phase rotations in excess of 90.degree., because the loop gain then becomes negative because of the characteristics of the sinusoid function of the mixers used in the loop. Such a phase rotation can be reached for example by a travelling wave tube of which the voltage between the electrodes varies by approximately 2%.
This latter solution can however be improved by adding a blocking sampler to the phase lock loop, to preset the correction to be made to the pulse to be processed according to the correction applied to the previous pulse.
The blocking sampler in fact stores the error signal on the pulse that has just been corrected and presets the phase shifter of the phase lock loop for the next correction. This improvement is still not enough, because it fails notably to deal with fluctuations between pulses, since it is in principle effective only for pulses that recur almost identically.
Moreover, such a blocking sampler is difficult to use and complicates adjustment.